This application claims the benefit of Application No. 10658/1996, filed in Korea on Apr. 9, 1996, which is hereby incoroprated by reference.
1. Field of the Invention
The present invention relates to a method for decoding MPEG (Moving Picture Expert Group) standard bit streams and, more particularly, to a method for decoding MPEG standard bit streams which requires less memory and can result in lower production costs.
2. Discussion of the Related Art
A conventional method for decoding MPEG standard video bit streams will be explained with reference to the attached drawings.
FIG. 1 is a block diagram of a system of a conventional device for decoding MPEG standard video bit streams. FIGS. 2a-2d are block diagrams showing frame memory structures. FIG. 3 illustrates a predictive structure between I, P and B picture data in a device for decoding an MPEG standard video bit stream
In general, each video frame data in an MPEG standard digital video system is specified in pixel units, of which a large amount of information requires a video compression (or encoding) technique to reduce the amount of information for efficient transmission or storage. This video compression technique mainly uses a technique of eliminating duplicated information located between video frames in spatial and time regions. The duplicity in the spatial regions comes from the minute rate of variation between adjoining pixels within a video frame. The duplicity in the time regions comes from the minute rate of variation between adjoining video frames, i.e., the variation of the movement of an object.
As is well known, an MPEG standard video bit stream is coded to be divided into three types of streams on the transmission side. The three types of streams are the I (Intra-coded) picture video bit stream, the P (predictive-coded) picture video bit stream, and the B (bidirectional predictive-coded) picture video bit stream. Among these bit streams, the most basic video bit stream is the I video bit stream. The I video bit stream is used as reference data for formation of the P video bit stream. The I video bit stream and the P video bit stream, in turn, are used as reference data for formation of the B video bit stream. Thus, compression of a motion picture is made possible. When the three compressed types of video bit streams are decoded at the reception side, the same relation exists with respect to the video bit streams as existed at the transmission side.
Referring to FIG. 1, a device for decoding general MPEG standard bit streams includes a memory 10 for receiving and storing the compressed three types of video bit streams, a controlling part 20 for controlling the decoding, and a decoder 30 for decoding the I, P, and B video bit streams using the aforementioned relation and for storing the decoded video frame data in the memory 10 in succession. The decoding device in FIG. 1 is connected to a display 40 for displaying on a screen the decoded video bit streams stored in the memory 10 under the control of the controlling part 20, a storage device 50 for storing the decoded video frame data stored in the memory 10 by the controlling part 20, and a transmission line for transmitting the decoded video frame data stored in the memory 10 by the controlling part 20.
Referring to FIG. 2a, the video frame memory 10 shown in FIG. 1 includes a first region 10a for storing the decoded I video frame data in succession (i.e., a video bit stream), a second region 10b for storing the decoded I video frame data, a third region 10c for storing the decoded P video frame data, and a fourth region 10d for storing the decoded B video frame data.
For implementation of a motion picture, the MPEG standard bit streams may only include two types of bit streams, i.e., the I and P picture bit streams. In this case, the fourth region 10d in the memory shown in FIG. 2a will not be used. Instead, only the first to third regions 10a-10c will be used. In this situation, the second region lob and the third region 10c shown in FIG. 2a may be represented as shown in FIG. 2d; the second region 10b may be divided into a region 10b1 for storing an up portion of the I picture video frame data and a region 10b2 for storing a down portion of the I picture video frame data; and the third region 10c may be divided into a region 10c1 for storing an up portion of the P picture video frame data and a region 10c2 for storing a down portion of the P picture video frame data.
In the conventional decoding method, upon reception of the compressed I, P and B picture video bit streams from the transmission side (for example, a broadcasting station), the memory 10 shown in FIG. 1 stores the video bit streams in succession. Under the control of the controlling part 20, the decoder 30 reads in the stored video bit streams in succession and decodes them into the I, P and B picture video frame data, which are then stored in respective regions 10b-10d of the memory 10 by the controlling part 20. As each of the second to fourth regions 10b-10d of FIG. 2b is adapted to store respective decoded video frame data, each of them has the capacity to store an entire set of data for one entire video frame. Once the I, P and B picture video frame data are completely stored in respective regions 10b-10d of the memory 10, the controlling part 20 displays them on the display 40, transfers them to another storage device 50 as the case demands, or transmits them through the transmission line for a transmission to another device. The decoder 30 refers to backward I picture video bit streams stored in the first region 10a, B picture video bit streams, and backward P picture video bit streams, when the decoder 30 decodes the P picture video bit stream.
When coded at the transmission side, each of the MPEG standard video bit streams has a system of configuration as shown in FIG. 2b. Referring to FIG. 2b, one of the video bit streams compressed according to the MPEG standard includes an up portion 60 and a down portion 70. Each of these portions, in turn, has a top field 60a, 70a and a bottom field 60b, 70b.
A sequence of a single image reproduction from one MPEG standard video bit stream having the aforementioned system or configuration will be explained with reference to FIG. 2c. FIG. 2c illustrates an example of an interlace scanning NTSC broadcasting system.
Referring to FIGS. 2b and 2c, scan lines 1, 3, . . . , 239 belonging to the top field 60a of the up portion 60 are first displayed on a screen. Scan lines 241, . . . , 477, and 479 belonging to the top field 70a of the down portion 70 are then displayed on the screen. Next, scan lines 2, 4, . . . , 240 belonging to the bottom field 60b of the up portion 60 are displayed on a screen. Then, scan lines 242, . . . , 478 and 480 belonging to the bottom field 70b of the down portion 70 are displayed on the screen. That is, as shown in FIG. 2c, the top field 60a of the up portion 60 and the top field 70a of the down portion 70 compose an odd field, and the bottom field 60b of the up portion 60 and the bottom field 70b of the down portion 70 compose an even field. As is well known, one odd field and one even field together compose one video frame. In addition, in interlace scanning, at first, odd scan lines are displayed on the screen and then even scan lines are displayed on the screen.
As for the aforementioned explanation, in the conventional decoding method, by the decoding of the decoder 30, each of the I, P, and B picture video frame data is obtained, all of which data are stored in the respective regions 10b-10d of the memory 10 and displayed by the controlling part 20 according to a predetermined sequence. Therefore, each of the regions 10b-10d of the memory 10 should have a capacity which can store all the video frame data of each picture.
FIG. 2d illustrates a diagram showing details of the second region 10b and the third region 10c in the memory 10 for the I and P picture video frame data shown in FIG. 2a.
Referring to FIG. 2d, in the memory 10, the second region 10b includes a portion 10b1 for storing the up portion, and a portion 10b2 for storing the down portion, of the I picture video frame data. Also, the third region 10c includes a portion 10c1 for storing the up portion, and a portion 10c2 for storing the down portion, of the P picture video frame data. The capacity of each of the portions corresponds to 25% of the capacity capable of storing all of the I and P picture video frame data.
The I, P and B picture video frame data, thus stored in the second to fourth regions 10b-10d of the memory 10, are either displayed by the display 40 according to a predetermined sequence, stored in the storage device 50, or transmitted to another device through the transmission line, by the controlling part 20.
FIG. 3 illustrates a system of reference illustrating the relationship between the MPEG standard I, P and B picture video frames in encoding.
In the MPEG standard, an I sequence of a motion picture is encoded in a plurality of video frames, i.e., in units of groups. Each such group includes a plurality of video frames. As has been explained, the plurality of video frames in each group includes I (Intra-coded) video frame data, P (Predictive-coded) video frame data, and B (Bidirectionally predictive-coded) video frame data.
The relationship of reference between these three video frame data is shown in FIG. 3.
Referring to FIG. 3, in each group, P picture video frame data are encoded with reference to their forward I picture video frame data, B picture video frame data are encoded with reference to their backward I picture frame data and their forward P picture video frame data. These frame data also have the same relations or relationships in decoding. The arrows in FIG. 3 indicate directions of the references. Since the I picture video bit stream is encoded only with information from itself, the compression ratio is not comparatively high. The P picture video bit stream is encoded referring to a backward I picture frame or other backward P picture frame. In this case, since the P picture video bit stream is encoded by synchronous compensation, a compression ratio higher than the I picture video bit stream can be obtained. However, because the P picture frame is encoded referring to a backward P picture frame at times, slight coding errors may occur. In the meantime, the B picture frame is encoded referring to backward and forward I and P pictures simultaneously, so that a highly compressed B picture video frame is obtainable. In the MPEG algorithm, the frequency and positions of I picture frames are selected dependent on a random accessibility or a frequency of scene change. In general, there are close correlations between video frames in a motion picture.
If all the MPEG standard I, P, and B picture video frame data are applied, the maximum size of one picture is about 1.49 Mbyte in the NTSC broadcasting system and 1.78 Mbyte in the PAL broadcasting system. Therefore, an adequate size of memory for those capacities is required in decoding. In addition to this, if the first region 10a shown in FIG. 2a, which is provided for storing received video bit streams, is taken into account, a size of memory having a capacity more than 2 Mbyte is required.
As has been explained, it is possible that the MPEG standard video bit streams may be encoded with only I and P picture video frames without B picture video frames. In this case, a memory of 0.99 Mbyte capacity is required for decoding in the NTSC broadcasting system, and a memory of 1.19 Mbyte capacity is required for decoding in the PAL broadcasting system. In this case, since only the I and P picture frame data are stored, a memory is required which has a capacity reduced by one frame data from a main profile that stores all the I, P, and B picture video frame data. However, since a memory having a capacity much greater than 1 Mbyte is still required, there has been a problem that production cost of a video appliance remains high.